i need the code that is similar to strlen(), but since i'm using cstring it will not work. I need the code which will count the number of characters in a variable, using a command which is not in string.h/

If your compiler is STL and namespace compliant then it probably also drops the h extension and uses the c prefix for the old standard files. Therefore to work with stuff in string.h in the new fangled compilers you need an include that looks like this:

#include <cstring>

Now how to communicate between the two. Well, there is little reason to do so, except for educational purposes because the vast majority of functions in cstring are methods in string as well. If you really want to use cstring functions on a string then you need to extract the underlying char[] in the string class using the c_str() method.